Woven glass electrical insulating sheet



June 1954 E. CRANDALL 2,679,577

WOVEN GLASS ELECTRICAL INSULATING SHEET Filed Oct. 50 1951 Patented June1, 1954 STATS QFFICE WOVEN GLASS ELECTRICAL mscmrmc SHEET New YorkApplication October 30, 1951, Serial No. 253,800

3 Claims. 1

This invention relates to insulating sheet material, more particularlyto insulating tape of the type comprising a fabric impregnated or coatedwith an insulating varnish, such as varnished linen or cotton fabric andnormally used by winding the tape helically on an electric conductorwith the adjacent turns of the tape in overlapping relation.

varnished cambric or varnished cloth is that particular type ofelectrical insulating material wherein the base fabric consists offibers (usually cotton) which are woven in a square weave andsubsequently coated and/or impregnated with a suitable electricalinsulating varnish to fill the voids between crossing threads of thefabric thereby to form a composite sheet.

After varnish treating the full width base cotton fabric (normally 37 to39 inches wide), the finished varnished cloth is subsequently slit intotape form of Widths varying from one-fourth inch to three inches. Thetape is wound helically around an electrical conductor such as a cableto form the required wall thickness of electrical insulation. Normalindustry practice requires that cloth tape be applied with an overlap toinsure the fact that the cable is completely covered. In this process itis obvious that the varnished cambric is being wound about two differehtdiameters; that is, at one edge of the tape there is a lap over thecontiguous tape, while at the opposite edge the tape is wrapped directlyon the cable. In order to apply such a tape smoothly without producingwrinkles, voids or edge tears, which are detrimental to the quality ofcable, it is necessary that the tape have the property of a certainpercentage of elongation or stretch. This is an inherent property in avarnished cambric tape because the base cotton fabric has a naturalresiliency giving it the property of a certain amount of elongation.

Cotton tape, however, while having the desirable stretching feature,does have a certain disadvantage in that it will absorb moisture andwill deteriorate if subjected to long time high temperature aging. Thereis a need in the art, therefore, to replace the cotton with fibers whichare not subject to moisture absorption and heat deterioration.

A further consideration is that, at times, the price of cotton becomesso high that it is economically necessary to have as an alternate a lessexpensive base material for electrical insulating tape.

While glass fibers for electrical insulation purposes with and withoutvarnish have been known,

2 for example see U. S. Patent No. 2,133,183, still the glass insulatingmaterial known heretofore would not fulfill the purpose intended by thisimproved invention since glass fibers do not have the natural elongationrequired for proper application about two different diameters.Consequently, voids, gaps and edge tears occurred in the prior art glassinsulation whereby the dielectric strength of the electrical insulatingmaterial is reduced with production of a substandard grade of cable.

It has also been suggested that the lack of elongation of glass fibresmight be circumvented by forming a yarn of a combination of glass fibresand asbestos fibres twisted together so that the glass fibres contributestrength to the yarn while the asbestos fibres contributestretchability. Such yarns, it was proposed, could be woven into fabricswhich could be treated with suitable insulating materials for electricalinsulation purposes. This proposal, however, has not fulfilled therequirements of the industry for a varnished cloth for use in place ofthe conventional varnished cambric for several reasons. Not only isadded cost and other disadvantages involved in combining several kindsof fibres into a yarn, but also by admixing other fibres with glassfibres, the desirable qualities of glass are lost to the extent of suchadmixing.

It is an object of this invention, therefore, to produce an electricalinsulating material utilizing a base fabric or glass fiber yarn so woventhat the finished product has the property of a certain percentage ofelongation or stretch whereby the material is adapted for use in cableor other electrical wrappings where a lapped type of winding isrequired.

It is a further object of this invention to provide an electricalinsulating material which is not subject to moisture absorption or heatdete'rioration.

It is a still further object of this invention to provide an improvedelectrical insulating material having greater insulating properties andflame resistance than heretofore known.

Further objects and advantages of this invention will become apparentand the invention will be more clearly understood from the followingdescription, referring to the accompanying drawing, and the features ofnovelty which characterize this invention will be pointed out withparticularity in the claims annexed to and forming a part of thisspecification.

Briefly, this invention relates to an improved type or electricalinsulating sheet material having a base of a woven glass fabric wherein,while an individual glass fiber yarn will not permit elongation, theweaving is such that the fabric will exhibit a significant longitudinalelongation. The preferred weaving is a leno or gauze weave wherein thelongitudinal or warp yarns are loosely twisted, so that when the fabricis longitudinally stretched the twist is tightened to give a certaindegree of over-all elongation.

For clarification and ease of reference, conventional insulatin materialwill be referred to by the usual industry term varnished cambric, whilemy new material, which is the subject matter of this invention, will bedesignated as varnished glass.

Referring to the drawing, Fig. 1 is a front elevation of a section ofcabl being wrapped with electrical insulatin tape; Fig. 2 is a magnifiedfront elevation of my improved varnished glass, showing the glass fabricin its normal position; while Fig. 3 is a magnified front elevation ofthe glass fabric in its stretched position.

Referring to the drawing, an electric cable I is shown in the process ofbeing wrapped with an electrical insulating tape 2 wherein adjacentedges of the tape are overlapped on contiguous tape to provideoverlapped portions 3. It is obvious that the overlapped portions 3 havea greater diameter than that of the cable itself. In order to compensatefor this overlapping portion, it is necessary that a certain degree ofelongation exist within the tape 2, whereby through such elongation atight casing is produced without voids or tears as the tape is wrappedabout the cable I.

The varnished glass tape 2, which is the subject matter of thisinvention, is made from a base glass fabric having a leno or gauze weavepattern (hereinafter explained). The glass fabric is then coated and/orimpregnated with an electrical insulating varnish to fill in all thegaps between the yarns of glass fibers thereby providing a continuoussheet of varnished glass. Preferably, as with the varnished cambric,tape 2 is formed as a sheet having any predetermined width which islater slit to desired widths, for example onefourth inch to threeinches. With such widths, the tape is adapted to be wrapped in rolls foreasy application to a cable.

Leno weave, by definition, is of the genus called crossed weaving. Thisgroup includes all fabrics such as gauzes, in which the warp threads oryarns t, 5 intertwist amongst themselves while the weft yarns or threadsare straight to give intermediate effects between ordinary weaving andlace. Plain gauze embodies the principles that underlie the constructionof all crossed woven textiles; that is, the twisting of two warp threadstogether leaves large interstices between both warp and weft. Butalthough light and open in texture, gauze fabrics are the firmest thatcan be made from a given quantity and quality of material. One warpthread 5 from each pair is made to cross the other between every pick,to the right and to the left alternately; therefore, the same threads 5are above every weft thread 6. In crossing from side to side, warpthreads 5 pass below the other warp threads 4, which in turn are belowevery weft thread 6; consequently, all are bound securely together asshown in Figs. 2, 3. Leno is heavier than gauze and it is composed of anodd number of picks of a plain weave followed by one pick of gauze.

It is important that the spacing of the longitudinal or warp threads 4,5 and the cross or weft threads 6 be so selected that when the glassfabric is varnish coated and/or impregnated with the electricalinsulating varnish, there will not be so great a stress placed on thevarnish film that it will break through the open mesh. Accordingly, thevarnish used with this varnished glass must form a film which willwithstand some elongation without failing electrically or mechanically.Ideally, such a varnish film would be a residue of one of the high gradeelectrical insulating varnishes available in the market wherein anunsupported film of varnish will withstand approximately 10% elongationwithout rupture or loss of electrical properties.

While it is not intended to be a limitation of the scope of thisinvention, it is to be noted that in one particular embodiment it hasbeen found that glass is so much stronger than cotton that a 24 x 12leno or gauze weave base glass fabric has adequate physical strength forcable application. However, tests on the finished varnished glass showthat this particular weave is too open to adequately support the varnishfilm. The 32 x 16 leno or gauze weave base glass fabric has proven moresatisfactory to this extent. However, other weaves, such as 32 x 20,etc., may be used. By 32 x 16 is meant 32 longitudinal or warp yarns and16 cross or weft yarns per square inch. With this structure the area ofthe interstices between the glass yarns is such that it will support avarnish film wherein an unsupported film of varnish will withstandapproximately 10% elongation without rupture or loss of electricalproperties.

It is to be noted that, due to the high physical strength of the glassyarn, a more open weave base glass fabric may be used in the manufactureof varnished glass as compared to varnished cambric. This attractivephysical property is of economic advantage in that the amount of threadused may be decreased as the weave is opened.

Preferably threads or yarns 4, 5 and 6 are of that yarn category knownin industry as -1/0. The 150-1/0 indicates a single end glass yarn withapproximately 15,000 yards per pound. This is a low cost material ascompared to other types of glass yarn and it is particularly suited tothe manufacture of a finished glass fabric of the requiredthickness-approximately 6 mils.

While glass fabric inherently has no elongation, and, therefore, whilethere would be no crosswise elongation due to the straight weft threads6, yet there is some longitudinal elongation of the varnished glass(which is actually a false stretching) due to the particular positioningof the yarns 4, 5: i. e., when the varnished material is cut into tapeit is cut lengthwise; i. e., parallel with the warp threads 4 and 5 sothat the tape can stretch lengthwise. By false stretching is meant thatas tension is produced on opposite ends of the warp yarns 4, 5, theytend to twist or tighten to the position shown in Fig. 3 wherebystretching of the varnished glass is produced even though there is noactual stretching of the individual threads of glass yarn.

It has been found that by using leno or gauze weave glass fabric such asthat described, an elongation in the order of 4 to 6 per cent isobtained, which is more than satisfactory for the use of the varnishedglass on cable or for other electrical wrappings where a lapped type ofwinding application is required. Tests have proven that this varnishedglass will lie flat about the cable and that there will be no voids,gaps or edge tears to destroy or lower the electrical or physicalproperties of the finished cable.

While in the above description reference has been made primarily to alapped tape wound around two different diameters, it is to be understoodthat the varnished glass may be used for butt joint type of applicationWhere the elongation of the tape is not necessary for normalapplication. However, the elongation in the varnished glass isbeneficial to the butt joint type of tape application, as the elongationof the tape allows bending of the cable without damage to theinsulation.

Modifications of this invention will occur to those skilled in the artand it is desired to be understood, therefore, that this invention isnot intended to be limited to th particular mbodiment disclosed, butrather it is intended to cover all modifications which are within thetrue spirit and scope of this invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. Insulating tape adapted for helical winding around a conductor withadjacent edges of successive turns overlapping comprising fabric of lenowoven glass yarn wherein the warp threads are loosely looped around thweft threads and extend longitudinally of the tape, and a resilientelectrical insulating varnish coating on the fabric capable ofstretching without rupture an amount sufiicient to permit of the tapebeing helically wound smoothly on a conductor with its edges overlappingwhereby when the tape is helically wound on a conductor with the tapeedges overlapping the overlapping tape edge can stretch sufliciently tocompensate for the increased diameter due to the overlap.

2, Insulating tape adapted for helical winding around a. conductor withadjacent edges of successive turns overlapping comprising fabric wovenfrom glass yarn wherein the weft threads extend transversely of thetape, and the warp threads extend longitudinally of the tape and arearranged in groups wherein all of the threads of each group are looselylooped across and are interlocked with the weft threads, each thread ofeach group crossing and recrossing at least one other thread in the samegroup, and a continuous film of resilient lectrical insulating varnishfilling the interstices between the threads, so that when the tape ishelically wound on a, conductor with the tape edges overlapping, theoverlapping tape edge can stretch sufliciently to compensate for theincreased diameter due to the overlap.

3. Insulating tape adapted for helical winding around a conductor withadjacent edges of successive turns overlapping comprising fabric wovenfrom glass yarn wherein the weft threads extend transversely of the tapeand the warp threads extend longitudinally of the tap and are arrangedin spaced pairs and wherein all of the warp threads are loosely loopedand are interlocked with the weft threads, the two threads in each paircrossing and recrossing each other, and a continuous film of resilientelectrical insulating varnish on both sides of the fabric, so that whenthe tape is helically wound on a conductor with the tape edgesoverlapping, the overlapping tape edge can stretch sufficiently tocompensate for the increased diameter due to the overlap.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 240,772 Schinneller Apr. 26, 1881 702,725 Hammesfahr June 12,1902 2,133,183 Baird et al. Oct. 1 1938 2,209,850 Shand et al. July 30,1940 2,360,245 McFarlane Oct. 10, 1944 FOREIGN PATENTS Number CountryDate 260,136 Great Britain Oct. 28, 1926

